As known, a bicycle bottom bracket assembly comprises a shaft and two crank arms associated with the opposite ends of the shaft. The shaft can be made in a distinct piece from the crank arms or can be made in a single piece with one of the two crank arms.
The component of the bicycle bottom bracket assembly consisting of a crank arm and a shaft coupled together or made in a single piece is typically identified with the expression “crank arm assembly.”
The bottom bracket assembly is mounted on the bicycle by housing the shaft in a housing box suitably provided in the bicycle frame. The rotation of the bottom bracket assembly with respect to the housing box is achieved by inserting a pair of rolling bearings onto the shaft. Each bearing is positioned on the shaft at a respective shaft body portion adjacent to the crank arm. When the bottom bracket assembly is mounted on the bicycle frame, each bearing is operatively arranged between the shaft and the housing box suitably provided in the frame.
Typically, the mounting system of the bearings on the shaft is configured so that the bearings, once mounted on the shaft, can provide a precise reference for the correct positioning of the bottom bracket assembly in the housing box provided in the bicycle frame and can allow possible variations in size of such a box and/or possible dilations of the shaft to be accommodated without compromising the correct positioning of the bottom bracket assembly with respect to the bicycle frame. However, the known bottom bracket assemblies are difficult to assemble and/or are not strong enough to resist high loads during exercise.